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Stilbite-Ca is a hydrated calcium sodium aluminum silicate mineral. It is characterized by its distinctive sheaf-like or radiating crystal aggregates, often forming bow-tie or fan-shaped clusters. Its color ranges from white to pink, salmon, yellow, brown, and sometimes colorless. It has a pearly to vitreous luster and is relatively soft. Stilbite-Ca is the most common and well-known member of the stilbite series, which also includes Stilbite-Na. The 'Ca' suffix indicates that calcium is the dominant extra-framework cation.
How to Identify
- Color
- White, colorless, pink, salmon, yellow, brown, reddish-brown. Often pale.
- Luster
- Vitreous to pearly, especially on cleavage surfaces.
- Texture
- Typically forms radiating, sheaf-like, or bow-tie aggregates. Individual crystals are tabular.
- Crystal Form
- Monoclinic, often forming tabular crystals elongated along [100], frequently twinned to produce characteristic sheaf-like or bow-tie aggregates. Also found as divergent, fibrous, or granular masses.
- Cleavage
- Perfect on {010}, yielding pearly luster on this surface.
- Geological Environment
- Commonly found in amygdaloidal cavities and veins within basaltic and other mafic volcanic rocks. Also occurs in hydrothermal veins, granites, gneisses, and as a diagenetic mineral in some sedimentary rocks.
Key Facts
- Hardness: 3.5 - 4 on the Mohs scale
- Specific Gravity: 2.12 - 2.22 g/cm³
- Crystal System: Monoclinic
- Color: White, colorless, pink, salmon, yellow, brown, reddish-brown
- Luster: Vitreous to pearly
- Transparency: Transparent to translucent
- Fracture: Uneven
- Cleavage: Perfect on {010}
- Composition: Hydrated calcium sodium aluminum silicate
Quick Check
- Color: White, pink, salmon, yellow, brown, colorless
- Luster: Vitreous to pearly
- Streak: White
Physical Characteristics
- Crystal Habit: Typically forms tabular crystals, often twinned into characteristic sheaf-like, bow-tie, or fan-shaped aggregates. Also found as divergent, fibrous, or granular masses.
- Cleavage Type: Perfect on {010}
- Fracture Type: Uneven
- Tenacity: Brittle
- Luster Type: Vitreous to pearly
Formation
Stilbite-Ca is a tectosilicate mineral belonging to the zeolite group. It typically forms as a secondary mineral in the amygdaloidal cavities and fractures of basaltic and other mafic volcanic rocks, where it precipitates from low-temperature hydrothermal solutions. It can also occur in gneisses, granites, and hydrothermal veins, and as a diagenetic mineral in some sedimentary rocks.
Usage
Stilbite-Ca has limited industrial uses. Like other zeolites, it has potential applications in ion exchange, molecular sieves, and catalysis, though other zeolites are more commonly employed. It is primarily valued as a collector's mineral due to its aesthetic crystal forms and colors.
Age Distribution
Formed in various geological periods, often associated with Cenozoic volcanic activity, but also found in older metamorphic and sedimentary rocks.
Where to Find
Deccan Traps, India
World-renowned for producing exceptional, large, and aesthetically pleasing Stilbite-Ca specimens, often associated with other zeolites like apophyllite and cavansite, in basaltic amygdales.
Iceland
Abundant in the basaltic lavas, forming excellent specimens in cavities.
Faroe Islands
Known for fine Stilbite-Ca specimens in volcanic rocks.
Nova Scotia, Canada
Occurs in basalt flows along the Bay of Fundy, often with other zeolites.
Patterson, New Jersey, USA
Found in the basalt quarries, producing good quality specimens.
Scotland, UK
Localities in the Inner Hebrides and other areas yield Stilbite-Ca in volcanic rocks.
Finding Tips
Look in Volcanic Rocks
Focus your search on areas with basaltic lava flows, especially in amygdaloidal cavities (gas bubbles filled with minerals) and fractures.
Associated Minerals
Stilbite-Ca often co-occurs with other zeolites (e.g., heulandite, laumontite, scolecite, analcime) and associated minerals like apophyllite, calcite, and quartz. Identifying these can lead to stilbite.
Examine Road Cuts and Quarries
Fresh exposures in volcanic terrains, such as road cuts, construction sites, and quarries, are excellent places to find zeolite minerals.
Check for Sheaf-like Aggregates
The distinctive radiating, sheaf-like, or bow-tie crystal habit is a key identifier for stilbite.
Observe Luster
The pearly luster on cleavage surfaces is a good indicator.
Similar Rocks
Heulandite
Heulandite-Ca
Also known as: Heulandite-Ca, Heulandite-Na, Heulandite-K
Laumontite
Laumontite-Ca
Also known as: Laumontite-Ca
Scolecite
Scolecite-Ca
Also known as: Scolecite-Ca
Analcime
Analcime
Also known as: Analcite
Scientific Classification
- Mineral Class
- Silicates
- Group
- Zeolite Group, Heulandite-Stilbite Series
- Crystal System
- Monoclinic
- Chemical Formula
- (Ca,Na,K)9[Al9Si27O72]·nH2O (simplified: CaAl2Si7O18·7H2O for Stilbite-Ca)
- Composition
- Hydrated calcium sodium aluminum silicate. The ideal end-member formula for Stilbite-Ca is NaCa4[Al8Si28O72]·30H2O, but it is often simplified to CaAl2Si7O18·7H2O for general description, with variable amounts of Na and K substituting for Ca.
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